This invention relates to the manufacture of an automotive catalytic converter comprising wound Fe-Cr-Al or Fe-Cr-Al-Y foil having an oxidized surface covered with densely spaced oxide whiskers and carrying a catalyst-impregnated alumina coating. More particularly, this invention relates to an improved method for applying gamma alumina powder suspended in an alumina gel to an alumina whisker-covered surface on oxidized metal foil to form a tightly adherent gamma alumina coating, suitable for supporting a catalyst.
U.S. patent application Ser. No. 098,075 filed Nov. 28, 1979, of common assignee-ownership, for which we and Charles W. Vigor are joint inventors, describes the manufacture of a monolith-type catalytic converter for treating automotive exhaust gases. The converter comprises corrugated metal foil suitably wound to form a structure having gas-conveying passages. The preferred foil is fabricated by a metal peeling process of a high-temperature, corrosion-resistant ferritic stainless steel alloy containing, by weight, 15 to 25% chromium (Cr); 3 to 6% aluminum (Al); optionally 0.3 to 1.0% yttrium (Y); and the balance iron (Fe). When the peeled alloy foil is heated in air under suitable conditions, densely spaced alumina whiskers grow on the surface, in marked contrast to conventional flat oxides. Although the whisker-covered surface exhibits at least a twelve-fold increase in surface area in comparison to the substrate geometric surface area (or the flat oxide), the whiskers do not provide a sufficient density of catalyst sites to optimize the converter efficiency per unit volume. Consequently, the oxidized foil surface is coated with a gamma alumina material capable of providing a 4,000.times. increase in surface area. The coating material comprises gamma alumina powder dispersed in an aqueous alumina gel that, upon drying, binds the powder into a cohesive layer suitable for supporting a noble metal catalyst. In addition to providing a high density of catalyst sites, the gamma alumina coating also prolongs the converter lifetime by reducing the detrimental effects of lead and some other exhaust gas constituents.
One problem generally encountered in wound foil converters concerns spalling of the catalyst-bearing gamma alumina coating. Spalling is fundamentally related to poor bonding between the applied alumina coating and the oxidized foil surface. Although the whiskers appear to provide a better base for gripping the coating than flat oxides, the desired spall resistance was not achieved when the powder-containing gel was sprayed directly onto the whiskered surface.
Therefore, it is an object of this invention to provide an improved method for applying a coating material comprising ceramic powder dispersed in an aqueous alumina gel onto an oxide whisker-covered surface to form a tightly adherent ceramic layer.
It is a more particular object of this invention to provide a method for coating a metal foil surface that is substantially covered with alumina whiskers with a spall-resistant gamma alumina layer applied as a dispersion of gamma alumina powder in an aqueous alumina gel. The method comprises priming the whisker-covered surface prior to applying the dispersed gamma alumina powder to strengthen the bonding between the whiskers and the layer without adversely affecting the high surface area and other properties of the gamma alumina. Thus, a metal foil carrying a gamma alumina coating is provided that is suitable for bearing a noble metal catalyst and thereby for forming a catalytic converter for treating automotive exhaust gases.